In many applications, it is useful to provide an electrical impedance transformation from an input having one impedance to an output having another electrical impedance. For example, in many communication devices, an antenna is used to receive signal and to transmit signals. The received signals are provided to a receiving amplifier of a receiver of the communication device. Moreover, the antenna may receive signals from a transmitter amplifier of a transmitter. Regardless of whether the transmission/reception of signals is half or full duplex, or even simplex, often times the antenna has an impedance that varies from the impedance of the amplifier (receiver or transmitter). As should be appreciated, mismatched impedances result in reflections and losses that are beneficially avoided.
Among other technologies, electrical impedance transformers can be based on bulk acoustic waves (BAW) devices. One type of electrical impedance transformer is based on a film bulk acoustic resonator (FBAR) structure. The transformer includes two acoustic stacks, each comprising a layer of piezoelectric material disposed between two electrodes. A decoupling material is disposed between the acoustic stacks. Acoustic waves achieve resonance across the acoustic stacks, with the resonant frequency of the waves being determined by the materials in the acoustic stack.
FBARs are similar in principle to bulk acoustic resonators such as quartz, but are scaled down to resonate at GHz frequencies. Because the FBARs have thicknesses on the order of microns, and length and width dimensions of hundreds of microns, FBARs beneficially provide a comparatively compact alternative to known resonators. However, certain known BAW-based electrical impedance transformers suffer from, among other drawbacks, insertion loss and reduced bandwidth.
There is a need, therefore, for an electrical impedance transformer that overcomes at least the shortcoming of known electrical impedance transformers discussed above.